Which is correct: "if I was" or "if I were"? And why?

The question of whether to use if I was or if I were is a question of mood, which, in grammarspeak, refers to the way in which a verb expresses an action or state of being. In the English language, sentences can take on three different moods:

Indicative:This is the most common mood and the easiest to understand. The indicative mood makes statements or asks questions: "My little brother is bugging me." "Is that tuna sandwich safe to eat?"

Imperative: An imperative statement makes requests or gives commands. The subject of an imperative sentence is often an understood "you" that does not actually appear in the sentence: "Don't bug your brother!" "Please put that tuna sandwich in the trash."

Subjunctive: A verb in the subjunctive mood deals with hypothetical situations or with ideas that are contrary to fact. "If my brother were a beetle, I would step on him." "If that tuna sandwich hadn't spoiled, I would've had a nice lunch."

In the present subjunctive, were is used for all people: "If I were a rich man . . . " "If she were only ten years younger . . . " "If they were only a bit more experienced . . . " The past subjunctive uses had in all cases: "If my brother hadn't been bugging me, I could have finished my homework." "If I hadn't eaten that sandwich, I wouldn't be in the hospital now." Note that the words would and could are good indicators of the subjunctive mood, although their appearance does not necessarily mean that a sentence is in the subjunctive mood.

Now, to come around to the original question, the subjunctive mood is the most common mood in the if I was/were dilemma, so if I were is more often (though not always) the way to go.

Not every if I statement should be in the subjunctive mood. Consider the following sentences:

If I was wrong, I apologize.

If I were wrong, I would apologize.

The first sentence is in the indicative mood — it actually offers up the speaker's apology. The second sentence, in the subjunctive mood, states either a) that an apology would be forthcoming if the speaker's error comes to light, or b) that the fact that the speaker hasn't offered an apology indicates that he or she was not wrong. In either case, in this second sentence, the speaker's error and apology are both hypothetical, and therefore the sentence is in the subjunctive mood.